1. Field of the Invention
The present invention relates to a filter circuit and an electronic device using the same, and more particularly, the present invention relates to a filter circuit including a distributed parameter RC circuit having a distributed parameter resistor and a distributed parameter capacitance, and a buffer circuit, and to an electronic device including such a filter circuit.
2. Description of the Related Art
An exemplary filter circuit having a distributed parameter resistor and a distributed parameter capacitance is disclosed in Japanese Unexamined Patent Application Publication No.8-265081, for example. FIG. 15 shows the exemplary filter circuit of this device. Further, a chip filter shown in FIG. 16, which uses the above-described filter circuit and has a notch characteristic, is disclosed in the same Japanese Unexamined Patent Application Publication. This chip filter 1 includes an internal electrode 3 disposed in a chip 2 which is defined by a dielectric material and two resistor layers 4a and 4b arranged so as to oppose the internal electrode 3.
Further, the chip filter 1 includes an electrode 5a connected to one end of the resistor layer 4a, an electrode 5b connected to the other end of the resistor layer 4a, an electrode 5c connected to one end of the resistor layer 4b and to the internal electrode 3, and an electrode 5d connected to the other end of the resistor layer 4b. FIG. 17 is the equivalent circuit diagram of the above-described chip filter 1. The chip filter 1 can adjust the filtering characteristic thereof by providing the resistor layer 4b with a slit as shown in FIG. 18.
FIG. 19 shows the configuration of another filter disclosed in Japanese Unexamined Patent Application Publication No. 10-93370. This filter 1 includes a dielectric layer 7a on one surface of a layer 6 and a dielectric layer 7b on the other surface of the layer 6. The filter 1 also includes electrode layers 8a and 8b sandwiching the dielectric layers 7a and 7b so that the electrode layers 8a and 8b oppose the resistor layer 6. FIG. 20 is the equivalent circuit diagram of the filter 1 including distributed parameter capacitances provided on both sides of a distributed parameter resistor. The filter circuit shown in FIG. 17 achieves a notch characteristic indicated by the dotted line shown in FIG. 21. However, the filter circuit shown in FIG. 20, which includes the distributed parameter capacitances on both sides of the distributed parameter resistor, can achieve a steep notch characteristic indicated by the solid line shown in FIG. 21. Further, the filter circuit having an active device as shown in FIG. 22 can achieve a notch characteristic that is steeper than in the above-described case.
The filter shown in FIGS. 16 and 18 is small in size and can adjust the filtering characteristic thereof by using a resistor layer having a slit formed on the chip. However, the filter cannot obtain a steep notch characteristic. More specifically, the attenuation of the higher frequency side adjacent to the notch is small. Although the filter can adjust the filtering characteristic thereof by using the resistor layer 4b having the slit shown in FIGS. 16 and 18, the adjustment is limited since the resistor thereof for defining the distributed parameter RC circuit has no slits.
Further, the filter shown in FIG. 19 cannot adjust the filtering characteristic thereof by using a resistor layer with a slit, since the resistor is disposed within the device. If the filter had a resistor layer with a slit, this type of distributed parameter filter has only one resistance. That is to say, the parameter of the filter for adjusting the filtering characteristic such as a cutoff frequency and the notch characteristic is limited.
In order to overcome the problems described above, preferred embodiments of the present invention provide a filter circuit which can achieve a steep notch characteristic and which has many parameters and thereby easily adjusts the filtering characteristic thereof, and an electronic device including such a novel filter circuit.
According to a first preferred embodiment of the present invention, a filter circuit includes n (n are positive numbers and nxe2x89xa72) distributed parameter RC circuits each having a distributed parameter resistor and a distributed parameter capacitance disposed along the distributed parameter resistor, and a buffer circuit. The distributed parameter resistors are connected in parallel at a first node and a second node, the second node is connected to an input terminal of the buffer circuit, and one to nxe2x88x921 distributed parameter capacitances of the n distributed parameter RC circuits are connected to an output terminal of the buffer circuit.
According to a second preferred embodiment of the present invention, a filter circuit includes a dielectric layer, distributed parameter resistors including n (nxe2x89xa72) resistor layers disposed on one surface of the dielectric layer, distributed parameter capacitances distributed along the distributed parameter resistors including n electrode layers arranged on the other surface of the dielectric layer so that the n electrode layers oppose the respective distributed parameter resistors, and a buffer circuit. In this filter circuit, first ends of the n resistor layers are electrically connected to each other and second ends of the n resistor layers are electrically connected to each other so that the distributed parameter resistors are connected in parallel. The second ends of the n connected resistor layers are connected to an input terminal of the buffer circuit, and one to nxe2x88x921 electrode layers are connected to an output terminal of the buffer circuit.
According to a third preferred embodiment of the present invention, an electronic device includes a filter circuit according to one of the preferred embodiments of the present invention described above.
FIGS. 23 and 24 show generally used filter circuits having a notch characteristic by using the distributed parameter RC circuit shown in FIG. 15. The transfer function F(s) of these circuits is shown by the following equation:       F    ⁡          (      s      )        =            α      +                        sCR                ⁢                  xe2x80x83                ⁢        sinh        ⁢                  sCR                                    α        ⁢                  xe2x80x83                ⁢        cosh        ⁢                  sCR                    +                        sCR                ⁢                  xe2x80x83                ⁢        sinh        ⁢                  sCR                    
wherein R indicates the overall resistance of the distributed parameter RC circuit, C indicates the overall capacitance thereof, and R0 and C0 respectively indicate the resistor and the capacitor.
Here, xcex1=R/R0, or xcex1=C/C0. According to the above equation, the filtering characteristic of the filter circuit shows the notch characteristic as indicated by the dotted line shown in FIG. 21. By replacing a single feedback obtained by the capacitor shown in FIG. 24 with a distributed parameter-type multiple feedback shown in FIG. 25, the steep notch characteristic indicated by the solid line shown in FIG. 21 can be obtained.
As has been described, the steep notch characteristic can be achieved by using the plurality of distributed parameter RC circuits connected in parallel and the buffer circuit. More specifically, for obtaining such steep notch characteristic, the n (nxe2x89xa72) distributed parameter RC circuits connected in parallel are used with the buffer circuit, for example. One end of each of n distributed parameter resistors is connected to the input terminal of the buffer circuit, and one to nxe2x88x921 distributed parameter capacitances are connected to the output terminal of the buffer circuit. Further, by adjusting the distributed parameter resistors, the filtering characteristic can be adjusted. Since the plurality of distributed parameter RC circuits is connected in parallel, there is the plurality of adjustable distributed parameter resistors. In other words, there are many parameters for performing fine adjustment of the filtering characteristic.
This filter circuit includes the plurality of resistor layers sandwiching the dielectric layer and the plurality of electrode layers. In this case, the filter circuit can adjust the resistance of the distributed parameter RC circuits by creating a slit in the resistor layers. Since the shape of the electrode layers and the resistor layers can be changed at will, it becomes easy to trim the filter circuit. Further, the electronic device including such a filter circuit can perform predetermined desired operations because of the suitable filtering characteristic of the filter circuit.
The above-described and other features, elements, characteristic, and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the drawings.